(1) Field of the Invention
The present invention relates to an image acquisition apparatus, such as a digital magnifying observation apparatus or a microscope that captures and displays an enlarged image.
(2) Description of the Related Art
Examples of an image acquisition apparatus that displays an object (e.g., a sample such as a minute substance, a workpiece, or the like) in an enlarged manner include an optical microscope, a digital microscope and the like each adopting an optical lens. In the digital microscope, a light receiving device such as a CCD that electrically reads an image for each of pixels arranged in a two-dimensional form receives light reflected from or transmitting through an observation target held by an observation target holding part onto which an optical system emits light, and a display part such as a display displays the image read electrically (refer to, e.g., JP 2006-030969 A).
Typically, the image acquisition apparatus described above is designed for the purpose of capturing an image of a still object to observe the still object. In other words, the image acquisition apparatus is unsuitable for capturing an image of a moving object. In a case where the image acquisition apparatus is used for diagnosing a cause of a malfunction occurring at a manufacturing line in a factory, for example, there is a need to successively capture images of a moving object. In this case, conventionally, a special-purpose high-speed image acquisition camera such as a high-speed camera has been used for successively capturing images. However, such a high-speed image pickup camera is expensive in general and is not used ordinarily. Consequently, use of the high-speed image pickup camera is not efficient in terms of cost, management and maintenance.
If the conventional image acquisition apparatus is used for capturing an image at a high speed, further, there arises a problem of an insufficient quantity of light. More specifically, if the image acquisition apparatus is used for the purpose of use for high-speed image capture, a working distance from the camera to the object becomes considerably long in comparison with a case where the image acquisition apparatus is used for the purpose of use for normal enlargement observation. Consequently, a quantity of light to be emitted to the object decreases inversely with a square of a distance. In addition, if the image capturing speed becomes higher, the quantity of the light relatively decreases. In the image acquisition apparatus, for example, the image capturing speed per one image is about 1/30th second as in the case of a normal video camera. On the other hand, the high-speed image pickup camera captures an image in an extremely short time in a range from 1/1000th second to 1/10000th or 1/1000000th second. For this reason, the high-speed image pickup camera requires a relatively high level of illuminance per unit frame. Conventionally, a separate illuminating fixture must be prepared independently of the camera. Herein, examples of the illuminating fixture include a light source capable of continuously emitting strong light, such as a halogen lamp or a metal halide lamp, a flash lamp capable of achieving high illuminance only in a short time, a light source emitting a pulse laser in synchronization with an image capturing period, and the like. However, such an illuminating fixture is expensive in general. In addition, operations such as installation of the illuminating fixture or positional adjustment of a target to be irradiated with light become burdensome to a user. Depending on types of light sources, further, a certain light source is still unsatisfactory in illuminance. Alternatively, another light source is satisfactory in illuminance, but is unsatisfactory in color rendering. Consequently, these light sources must selectively be used in accordance with the purpose of use. However, such a light source is of a fixed type in general; therefore, an operation for exchanging the light source becomes burdensome to a user. Hence, a plurality of illuminating fixtures different in types from one another must be prepared, resulting in further increase of cost and effort.
In addition, when a large quantity of illumination light is attempted to be obtained for an object having a long working distance, a center portion of the illumination light becomes dark, so that the quantity of illumination light decreases. Consequently, there arises a problem of so-called hollow defective illumination. As shown in FIG. 23, more specifically, a light collecting lamp directly collects light for an optical fiber 61A. In this configuration, since a light emitting tube itself of a light source serves as a shade, the light emitted from the lamp is reflected by a reflector and, then, is made incident onto the optical fiber 61A in directions shown by arrow marks A and B. When the light is emitted toward a farther side through the optical fiber 61A, a center of emission becomes dark at the farther side, that is, hollow defective illumination occurs at the farther side due to lack of an optical component parallel with an optical axis. Consequently, the problem of hollow defective illumination becomes obvious with regard to the object having the large working distance. Hence, the object can not be irradiated with a satisfactory quantity of illumination light with ease.